Organic electroluminescent device based upon emission of exciplexes or electroplexes, and a method for its fabrication

ABSTRACT

An organic electroluminescent device ( 1 ) based upon emission of exciplexes or electroplexes, the device basically including an anode ( 2 ), a cathode ( 3 ), a first layer ( 4 ), which comprises organic material for transporting positive charges ( 5 ), and a second layer ( 6 ), which comprises organic material for transporting negative charges ( 7 ), said organic material for transporting negative charges ( 7 ) and said organic material for transporting positive charges ( 5 ) being capable to form between them exciplexes or electroplexes.

TECHNICAL FIELD

The present invention relates to an organic electroluminescent devicebased upon emission of exciplexes or electroplexes with high emissionefficiency.

BACKGROUND ART

In the field of organic electroluminescent devices (OLEDs) there haverecently been proposed organic electroluminescent devices that useexciplexes, which are formed by a material for transporting negativecharges and by a material for transporting positive charges, for theemission of light radiation. In particular, the use is known ofelectroluminescent devices comprising an anode and a cathode, betweenwhich is set an intermediate layer of organic material, which comprisesa mixture of the organic material for transporting positive charges andof the organic material for transporting negative charges. Althoughfurther embodiments of this type of devices envisage the insertion offurther layers of organic material, the presence of the intermediatelayer, inside which the exciplexes are formed, has always beenconsidered essential for the functioning of this type of OLEDs.

The presence of the mixed intermediate layer, between the anode and thecathode renders devices of this type costly and difficult tomanufacture, in particular, in view of the fact that the intermediatelayer is usually obtained by means of a relatively complex and somewhatdifficult operation, namely a simultaneous sublimation of two substanceshaving physico-chemical characteristics that are different from oneanother.

DISCLOSURE OF INVENTION

The purpose of the present invention is to provide an organicelectroluminescent device, which is free from the drawbacks describedabove and is, hence, easy and inexpensive to manufacture.

According to the present invention, there is provided an organicelectroluminescent device based upon emission of exciplexes orelectroplexes, the organic electroluminescent device essentiallyincluding an anode, a cathode, a first layer, which comprises at leastone organic material for transporting positive charges and is set incontact with the anode, and a second layer, which comprises at least oneorganic material for transporting negative charges and is set in contactwith said cathode and with said first layer, said organic material fortransporting negative charges and said organic material for transportingpositive charges being capable to form between them exciplexes orelectroplexes.

Here and in the ensuing text, the expression “essentially including”does not mean that the organic electroluminescent device cannot includeother constituents, but means that there is not present between theanode and the cathode a layer that comprises a mixture of the organicmaterial for transporting negative charges and of the organic materialfor transporting positive charges.

In the device defined above, it is possible that leakage currents willbe created, which do not contribute to the emission of theelectromagnetic radiation and are due, above all, to positive currents(i.e., a transfer of holes between adjacent molecules) that start fromthe anode, traverse the first and the second layer, and discharge at thecathode. The passage of charge between the first and second layersoccurs as a consequence of an electron jump from the HOMO of the organicmaterial for transporting negative charges to the HOMO (in which an holeis present) of the organic material for transporting positive charges.These currents, in addition to diminishing the efficiency of the OLED,raise the temperature, causing morphological alterations of the firstlayer and of the second layer, with consequent damage to the device.

For the above reason, preferably, said organic material for transportingnegative charges has a first ionization potential and said organicmaterial for transporting positive charges has a second ionizationpotential, the first ionization potential being higher by at least 0.7electronvolts than the second ionization potential.

Furthermore, it is possible, albeit with relatively less likelihood,that leakage currents will be created, which do not contribute to theemission of the electromagnetic radiation and are due above all tonegative currents (i.e., passage of electrons between adjacentmolecules) that start from the cathode, traverse the second and firstlayers, and discharge at the anode. The passage of charge between thesecond and first layers occurs, in this case, as a consequence of anelectron jump from the LUMO of the organic material for transportingnegative charges to the LUMO of the organic material for transportingpositive charges.

Also the negative currents, in addition to diminishing the efficiency ofthe OLED, raise the temperature, causing morphological alterations ofthe first and second layers, with consequent damage to the device.

Consequently, according to a preferred embodiment, said organic materialfor transporting negative charges has a first electronic affinity andsaid organic material for transporting positive charges has a secondelectronic affinity, the first electronic affinity being higher by atleast 0.4 electronvolts than the second electronic affinity.

The present invention moreover relates to a method for the fabricationof an organic electroluminescent device.

According to the present invention, a method is provided for thefabrication of an organic electroluminescent device based upon emissionof exciplexes or electroplexes, the method basically including the stepsof: depositing on an anode a first layer comprising at least one organicmaterial for transporting positive charges; depositing on said firstlayer a second layer comprising an organic material for transportingnegative charges; and positioning on said second layer a cathode, theorganic material for transporting negative charges and the organicmaterial for transporting positive charges being capable to form betweenthem exciplexes or electroplexes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the annexeddrawings, which illustrate some non-limiting examples of embodiment, inwhich:

FIG. 1 is a cross section of a first embodiment of the device accordingto the present invention;

FIG. 2 is a perspective view, with parts removed for reasons of clarity,of a detail of a second embodiment of the device according to thepresent invention;

FIG. 3 illustrates a spectrum of emission of a device built according toExample 1;

FIG. 4 is an experimental graph representing the function intensity ofelectroluminescence vs. applied voltage, and the function currentdensity vs. applied voltage of a device built according to Example 1;and

FIG. 5 is an experimental graph representing the function efficiency ofa device vs. applied voltage of a device built according to Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIG. 1, the number 1 designates as a whole an organicelectroluminescent device comprising an anode 2 and a cathode 3 that areseparated from one another by a layer 4 of an organic material fortransporting positive charges 5 and by a layer 6 of an organic materialfor transporting negative charges 7, which are in contact with oneanother, but substantially completely separated. The organic materialfor transporting positive charges 5 is capable to combine with theorganic material for transporting negative charges 7 so as to formexciplexes or electroplexes, which, by decaying from one of theirelectrically excited states, are able to emit electromagnetic radiation.

The cathode 3 and the anode 2 are connected (in a known way and hereschematically illustrated) to an external current generator 8, which isdesigned to induce a difference of potential between the cathode 3 andthe anode 2.

The layer 4 is designed to transfer holes, which are caused, in use, bythe oxidative processes that occur at the anode 2, from the anode 2towards the layer 6. The layer 4 is set in contact with the anode 2itself and with the layer 6, so as to be positioned on the opposite sideof the cathode 3 with respect to the layer 4.

The layer 6 is designed to transfer electrons coming from the cathode 3towards the layer 4 and is set in contact with the cathode 3 and on theopposite side of the anode 2 with respect to the layer 4.

A glass substrate 9 is set on the opposite side of the layer 4 withrespect to the anode 2 and provides a mechanical support to the anode 2,which has a relatively thin layer of a material with high work function,for example indium and tin oxides (ITOs). In this connection, it isimportant to emphasize that both the anode 2 and the glass substrate 9,since they are transparent, enable passage of light.

The cathode 3 is provided with a relatively thin layer, which is made ofa material with low work function, for example calcium, and is setunderneath a layer of silver 10.

In use, the current generator 8 is actuated so as to generate adifference of potential between the anode 2 and the cathode 3. The holesthat are created at the anode 2 in the material for transportingpositive charges 5 transfer on account of the electric field generatedbetween the cathode 3 and the anode 2 up to an interface 11, defined bythe layers 4 and 6. Likewise, the electrons transferred from the cathodeto the material for transporting positive charge 7 transfer through thelayer 6 as far as the interface 11.

At this point, the molecular cations of the layer 4 and the molecularanions of the layer 6 combine at the interface so as to form exciplexesor electroplexes, i.e., a combination of at least two molecules in anexcited state, which decay, dissociating to form the constituentmolecules and emitting electromagnetic radiation.

From what has been set forth above, it emerges that the selection of theorganic materials for transporting negative charges and for transportingpositive charges must be carried out with care. In particular, theorganic materials for transporting positive charges 5 and the materialfor transporting negative charges 7 must be chosen so as to be able toform between them exciplexes or electroplexes.

In order to improve the efficiency of the organic electroluminescentdevice 1, it is preferable for the organic material for transportingnegative charges to have the ionization potential higher by at least 0.7electronvolts than the ionization potential of the organic material fortransporting positive charges. In this way, the electrons present on theHOMO of the organic material for transporting negative charges 7, whichis set at the interface 11, basically do not succeed in passing onto theHOMO of the organic material for transporting positive charges 5, whichis set at the interface 11.

It is moreover preferable for the electronic affinity of the organicmaterial for transporting negative charges 7 to be higher by at least0.4 electronvolts than the electronic affinity of the organic materialfor transporting positive charges 5. In a way similar to what occurs inthe case of the positive charges, in this way, the electrons coming fromthe cathode present on the LUMO of the material for transportingnegative charges 7, which is set at the interface 11, basically fail topass onto the LUMO of the organic material for transporting positivecharges 5, which is set at the interface 11.

By so choosing the organic materials for transporting negative charges 7and positive charges 5, leakage currents, which do not contribute to theemission of electromagnetic radiation, are substantially limited.

Preferably, the organic material for transporting negative charges 7 isselected in such a way that its electronic affinity will be relativelyclose to the work function of the material of which the cathode issubstantially made, and the material for transporting positive charges 5is selected in such a way that its ionization potential will berelatively close to the work function of the material of which the anodeis substantially made.

The organic material for transporting positive charges 5 preferablycomprises a tertiary aromatic amine which is suitable to transferpositive charges and satisfies the structural formula (I):

in which T¹ and T² represent, each independently of the other, atertiary amine, and in which A represents an aryl group. By theexpression “each independently of the other” is meant the fact that T¹and T² can be identical to one another or different.

Preferably, T¹ and T² represent, each independently of the other, atertiary amine that satisfies the structural formula (II) or thestructural formula (III):

in which R¹ and R², represent, each independently of the other, an alkylgroup, an alcohol group, or an atom of hydrogen; and in which Ar¹ andAr² represent, each independently of the other, an aryl group.

Ar¹ and Ar², preferably, represent, independently of one another, afunctionality that satisfies one the structural formulas (IV), (V),(VI), (VII), (VIII), (IX) or (X):

in which R³, R⁴, R⁵, R⁶, R⁷, R⁹, R¹⁰ and R¹¹ represent, eachindependently of the others, an alkyl group, an alcohol group, or anatom of hydrogen, and in which S¹, S², S³ and S⁴ represent, eachindependently of the others, the functionality (XI), (XII), (XIII), or(XIV):

in which R¹², R¹³, R¹⁴, R¹⁵, R¹⁶, and R¹⁷ represent, each independentlyof the others, an alkyl group, an alcohol group, or an atom of hydrogen.

The organic material for transporting negative charges 7 comprises,preferably, an oxidiazole that satisfies the structural formula (XV) ora triazole that satisfies the structural formula (XVI):

in which E¹, E², E³, E⁴ and E⁵ are, each independently of the others, anaryl group.

E¹, E², E³, E⁴ and E⁵ preferably represent, each independently of theothers, a substituent that satisfies the structural formula (XVII),(XVIII), (XIX), (XX), (XXI), (XXII), (XXIII), (XXIV), (XXV), (XXVI), or(XXVII):

in which R¹⁸, R¹⁹, R²⁰, R²¹, R²², R²³, R²⁴, and R²⁵ represent, eachindependently of the others, an alkyl group, an alcohol group, or anatom of hydrogen, and in which S⁵, S⁶, S⁷, S⁸, S⁹, S¹⁰ S¹¹, S¹², S¹³,S¹⁴, S¹⁵, S¹⁶ and S¹⁷ represent, each independently of the others, afunctionality that satisfies the structural formula (XXVIII), (XXIX),(XXX), or (XXXI):

in which R²⁵, R²⁶, R²⁷, R²⁸, R²⁹, and R³⁰ represent, each independentlyof the others, an alkyl group, an alcohol group, or an atom of hydrogen.

The variant illustrated in FIG. 2 relates to an organicelectroluminescent device 12 similar to the device 1 and the parts ofwhich are designated by the same reference numbers that designate thecorresponding parts of the control device 1.

The device 12 differs from the device 1 substantially in that, in thedevice 12, there are present a plurality of anodes 2 and of cathodes 3having the shape of a parallelepiped with a rectangular base, thecathodes 3 lying on a plane that is different from and parallel to theplane on which the anodes 2 lie. The layers 4 and 6 are set between thetwo planes. The longitudinal axes of the cathodes 3 are parallel to oneanother and transverse to the longitudinal axes of the anodes 2. In thisway, the cathodes 3, by being set on top of the anodes 2, define aplurality of areas 13, each of which can light up individually andindependently of the others.

Further characteristics of the present invention will emerge from theensuing description of some non-limiting examples of the organicelectroluminescent device 1.

EXAMPLE 1

An organic electroluminescent device was prepared in the following way.

A plate of glass coated with a layer of indium and tin oxide, which hasa thickness of approximately 100 nm and is substantially transparent,was cleaned by being dipped in a boiling solution of acetone and alcoholand subsequently being put into an ultrasound washer for approximatelythirty minutes.

At this point, the following layers were deposited, in succession, oneon top of the other, by sublimation in an high-vacuum evaporator and ata pressure of 8×10^(□4) Pa, on the coated glass plate: a layer of4,4′,4″-tri(N,N-diphenyl-amino)-triphenylamine (TDATA) of a thickness of60 nm; a layer of a thickness of 60 nm of3-(4-diphenylyl)-4-phenyl-5-ter-butylphenyl-1,2,4-triazole (PBD); alayer of calcium of a thickness of 25 nm; and a layer of silver of athickness of 100 nm.

Note that the ionization potential and the electronic affinity of TDATAare substantially between 5 eV and 5.1 eV and 1.5 eV and 1.9 eV,respectively. The ionization potential and the electronic affinity ofPBD are approximately 6.3 eV and 2.8 eV, respectively. Consequently, inabsolute value, the differences between the potentials of ionization andbetween the electronic affinities of TDATA and of PBD are approximately1.2 eV and 1.1 eV, respectively.

The device thus obtained, which has an active surface of 0.07 cm², wastested under laboratory conditions (i.e., with a temperature of between20° C. and 24° C. and with a humidity of between 55% and 65%) andrevealed an electromagnetic emission in the green having a spectrum asis illustrated in FIG. 3. The curves that are obtained experimentallyfrom the use of said device and which represent the intensity ofelectroluminescence and the current density as a function of the voltageapplied are illustrated in FIG. 4. The curve that is experimentallyobtained from the use of said device representing the efficiency as afunction of the applied voltage is illustrated in FIG. 5.

EXAMPLE 2

An organic electroluminescent device was prepared in a substantiallyidentical way as the organic electroluminescent device of Example 1,except for the fact that, instead of the layer of TDATA, a layer of4,4′,4″-tri(carbazol-9-yl)-triphenylamine (TCTA) was deposited.

Note that the ionization potential and the electronic affinity of TCTAare approximately equal to 5.6 eV and 2.3-1.9 eV, respectively. Theionization potential and the electronic affinity of PBD areapproximately 6.3 eV and 2.8 eV, respectively. Consequently, in absolutevalue, the differences between the potentials of ionization and betweenthe electronic affinities of TCTA and PBD are approximately 0.7 eV and0.5 eV, respectively.

The device thus obtained, which has an active surface of 0.07 cm², wastested under laboratory conditions (i.e., with a temperature of between20° C. and 24° C. and with a humidity of between 55% and 65%) andrevealed an electromagnetic emission in the blue-violet.

EXAMPLE 3

An organic electroluminescent device was prepared in a substantiallyidentical way as the organic electroluminescent device of Example 2except for the fact that, instead of the layer of TCTA, there wasdeposited a layer of4,4′,4″-tri(N-3-methylphenyl-N-phenyl-amino)-triphenylamine (M-TDATA).Note that the ionization potential and the electronic affinity ofM-IDATA are substantially between 5 eV and 5.1 eV and 1.5 eV and 1.9 eV,respectively. The ionization potential and the electronic affinity ofPBD are approximately 6.3 eV and. 2.8 eV, respectively.

Consequently, in absolute value, the differences between the potentialsof ionization and between the electronic affinities of M-TDATA and ofPBD are approximately 1.2 eV and 1.1 eV, respectively.

The device thus obtained, which has an active surface of 0.07 cm², wastested under laboratory conditions (i.e., with a temperature of between20° C. and 24° C. and with a humidity of between 55% and 65%) andrevealed an electromagnetic emission in the green substantiallyidentical to the emission of Example 1.

EXAMPLE 4

An organic electroluminescent device was prepared in a substantiallyidentical way as the organic electroluminescent device of Example 1,except for the fact that, instead of the layer of TDATA, there wasdeposited a mixed layer of TDATA and polycarbonate.

The device thus obtained, which has an active surface of 0.07 cm², wastested under laboratory conditions (i.e., with a temperature of between20° C. and 24° C. and with a humidity of between 55% and 65%) andrevealed an electromagnetic emission in the green that was substantiallyidentical to the emission of Example 1.

EXAMPLE 5

An organic electroluminescent device was prepared in a substantiallyidentical way as the organic electroluminescent device of Example 1,except for the fact that, instead of the layer of PBD, there wasdeposited a layer of 3.5-bi(4-ter-butyl-phenyl)-4-phenyl-triazole (TAZ).

The device thus obtained, which has an active surface of 0.07 cm², wastested under laboratory conditions (i.e., with a temperature of between20° C. and 24° C. and with a humidity of between 55% and 65%) andrevealed an electromagnetic emission in the green.

1. An organic electroluminescent device (1) based upon emission ofexciplexes or electroplexes, said organic electroluminescent device (1)basically including an anode (2), a cathode (3), a first layer (4),which comprises at least one organic material for transporting positivecharges (5) and is set in contact with the anode (2), and a second layer(6), which comprises at least one organic material for transportingnegative charges (7) and is set in contact with said cathode (3) andwith said first layer (4), said organic material for transportingnegative charges (7) and said organic material for transporting positivecharges (5) being capable to form between them exciplexes orelectroplexes.
 2. The device of claim 1, wherein said anode (2) issubstantially transparent.
 3. The device of claim 2, and comprising atransparent substrate (9) set in contact with said anode (2).
 4. Thedevice of claim 2, wherein said anode (2) comprises indium and tinoxides (ITOs).
 5. The device of claim 3, wherein said transparentsubstrate (9) is a sheet of glass.
 6. The device of claim 1, whereinsaid organic material for transporting negative charges (7) has a firstionization potential, and said organic material for transportingpositive charges (8) has a second ionization potential, said firstionization potential being higher by at least 0.7 electronvolts than thesecond ionization potential.
 7. The device of claim 1, wherein saidorganic material for transporting negative charges (7) has a firstelectronic affinity, and said organic material for transporting positivecharges (5) has a second electronic affinity, said first electronicaffinity being higher by at least 0.4 electronvolts than said secondelectronic affinity.
 8. The device of claim 1, wherein said material fortransporting positive charges (5) is substantially made up of a tertiaryaromatic amine for transporting positive charges, said tertiary aromaticamine satisfying the structural formula:

in which T¹ and T² represent, each independently of the other, atertiary amine, and in which A represents an aryl group.
 9. The deviceof claim, wherein T¹ and T² represent, each independently of the other,a tertiary amine that satisfies a structural formula chosen in the groupconsisting of:

in which R¹ and R², represent, each independently of the other, onechosen from among: an alkyl group, an alcohol group, or an atom ofhydrogen. in which Ar¹ and Ar² represent, each independently of theother, an aryl group.
 10. The device of claim, wherein Ar¹ and Ar²represent, each independently of the other, a functionality thatsatisfies a structural formula chosen in the group consisting of:

in which R³, R⁴, R⁵, R⁶, R⁷, R⁹, R¹⁰ and R¹¹ represent, eachindependently of the others, one chosen from among: an alkyl group, analcohol group, or an atom of hydrogen; and in which S¹, S², S³ and S⁴represent, each independently of the others, a functionality chosen inthe group consisting of:

in which R¹², R¹³, R¹⁴, R¹⁵, R¹⁶, and R¹⁷ represent, each independentlyof the others, one chosen from among: an alkyl group, an alcohol group,or an atom of hydrogen.
 11. The device of claim 8, wherein A representsan aryl group that satisfies a structural formula chosen in the groupconsisting of:


12. The device of claim 8, wherein said tertiary aromatic amine ischosen in the group consisting of4,4′,4″-tri(N-3-methylphenyl-N-phenyl-amino)-triphenylamine (M-TDATA),4,4′,4″-tri(N,N-diphenyl-amino)-triphenylamine (TDATA), or4,4′,4″-tri(carbazol-9-yl)-triphenylamine (TCTA).
 13. The device ofclaim 1, wherein said material for transporting negative charges (7) isessentially made up of a heterocyclic compound that satisfies one chosenfrom among the structural formulas:

in which E¹, E², E³, E⁴ and E⁵ represent, each independently of theothers, an aryl group.
 14. The device of claim 13, wherein E¹, E², E³,E⁴ and E⁵ represent, each independently of the others, a substituentthat satisfies one chosen from among the following structural formulas:

in which R¹⁸, R¹⁹, R²⁰, R²¹, R²², R²³, R²⁴, and R²⁵ represent, eachindependently of the others, one chosen from among: an alkyl group, analcohol group, or an atom of hydrogen; and in which S⁵, S⁶, S⁷, S⁸, S⁹,S¹⁰ S¹¹, S¹², S¹³, S¹⁴, S¹⁵, S¹⁶ and S¹⁷ represent, each independentlyof the others, a functionality that satisfies one chosen from among thefollowing structural formulas:

in which R²⁵, R²⁶, R²⁷, R²⁸, R²⁹, and R³⁰ represent, each independentlyof the others, one chosen from among: an alkyl group, an alcohol group,or an atom of hydrogen.
 15. The device of claim 13, wherein saidheterocyclic compound is chosen in the group consisting of:3,5-bi(4-ter-butyl-phenyl)-4-phenyl-triazole (TAZ), or3-(4-diphenylyl)-4-phenyl-5-ter-butylphenyl-1,2,4-triazole (PBD). 16.The device of claim 1, in which said cathode (3) comprises a metalchosen in the group consisting of: alkaline metals, or alkaline-earthmetals.
 17. A method for the fabrication of an organicelectroluminescent device (1) based upon emission of exciplexes orelectroplexes, said method including basically the steps of: depositingon an anode (2) a first layer (4) comprising at least one organicmaterial for transporting positive charges (5); depositing on said firstlayer (4) a second layer (6) comprising an organic material fortransporting negative charges (7); positioning on said second layer (6)a cathode (3), said organic material for transporting negative charges(7) and said organic material for transporting positive charges (5)being capable to form between them exciplexes or electroplexes.
 18. Thedevice of claim 17, wherein said organic material for transportingpositive charge (5) and of said organic material for transportingnegative charge (7) are chosen so as to obtain selectively a pre-setwavelength of the emission of exciplexes or electroplexes.
 19. Thedevice of claim 17, and comprising the step of positioning said anode(2) on a transparent substrate (9).
 20. The device of claim 17, whereinsaid organic material for transporting negative charges (7) has a firstionization potential and said organic material for transporting positivecharges (5) has a second ionization potential, said first ionizationpotential being higher by at least 0.7 electronvolts than said secondionization potential.
 21. The device of claim 17, wherein said organicmaterial for transporting negative charges (7) has a first electronicaffinity and said organic material for transporting positive charges (5)has a second electronic affinity, said first electronic affinity beinghigher by at least 0.4 electronvolts than said second electronicaffinity.
 22. The device of claim 17, wherein said material fortransporting positive charges (5) is substantially made up of a tertiaryaromatic amine for transporting positive charges, said tertiary aromaticamine satisfying the structural formula:

in which T¹ and T² represent, each independently of the other, atertiary amine, and in which A represents an aryl group.
 23. The deviceof claim 17, wherein said material for transporting negative charges (7)is substantially made up of a heterocyclic compound that satisfies onechosen from among the structural formulas:

in which E¹, E², E³, E⁴ and E⁵ are, each independently of the others, anaryl group.